Optical Pickup Having Radially Arranged Lenses in a Low Profile Construction

ABSTRACT

An optical pickup includes a first source which emits a first beam with a first wavelength; a second source which emits a second beam with a wavelength shorter than the first wavelength; a first collimate lens which collimates the first beam; a second collimate lens which collimates the second beam; a first objective lens which converges the first collimated beam onto an optical disc; and a second objective lens which converges the second collimated beam onto the disc. The first and second objective lenses are arranged in the disc radial direction. The second objective lens is arranged closer to the side of the disc outer circumference than the first objective lens. The first collimate lens is arranged on the right-hand side when the second objective lens is viewed from the first objective lens. The second collimate lens is arranged on the left-hand side when the first objective lens is viewed from the second objective lens. The gap between the first collimate lens and the first objective lens is larger than the gap between the second collimate lens and the second objective lens.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. patentapplication Ser. No. 11/712,184, filed on Feb. 27, 2007, which claimspriority from Japanese Application JP-2006-283246, filed on Oct. 18,2006, the content of which is hereby incorporated by reference into thisapplication.

BACKGROUND OF THE INVENTION

The present invention relates to an optical pickup and an optical discdrive.

As a background art of the present technical field, an actuator mountingtwo objective lenses in the optical disc radial direction is disclosedin Japanese Patent Laid-open No. 2003-281758 (hereinafter referred to asReference 1). A means for downsizing two objective lenses mounted on anoptical pickup is disclosed in Japanese Patent Laid-open No. 2006-172610(hereinafter referred to as Reference 2). An optical pickup which mountstwo objective lenses in the optical disc radial direction is disclosedin Optical Data Storage 2006 Topical Meeting Conference Proceedings WPD3(FIG. 2 in Clause 33) (hereinafter referred to as Reference 3).

SUMMARY OF THE INVENTION

In recent years, as an optical disc, compact disc (CD), digitalversatile disc (DVD), and Blu-ray disc (BD) have been available. Each ofCD, DVD, and BD optical disc drives has a different wavelength of lightemitted from a laser light source, a different thickness of a coverlayer of an optical disc, and a different numerical aperture (NA) ofobjective lens. An optical pickup for CD and DVD generally uses only oneobjective lens because the wavelength, NA, and cover layer thicknesshave been compensated through the use of an objective lens ofdiffraction type.

With an optical pickup for CD, DVD, and BD, however, using only oneobjective lens causes many challenges with respect to the performancealthough it is not unrealizable. When an objective lens of diffractiontype is used to compensate the wavelength, NA, and cover layerthickness, the objective lens must have a deep-groove structure,resulting in a significantly degraded transmission efficiency of theobjective lens. In particular, the degradation is significant for CD,and the transmission efficiency of the objective lens is almost halved.Therefore, it is necessary that light be emitted from the laser lightsource with power that is at least twice the power in conventionalcases. Thus, high-power laser emission from the laser light source notonly shortens the life of the laser light source but also increases theheat release of the optical disc drive. Accordingly, a cooling structureneeds to be improved, which is disadvantageous for downsizing.Furthermore, an optical beam not transmitted by the objective lensbecomes unnecessary light which enters a Photo Detector (PD). In thiscase, challenges arise such as degraded reading performance and servosignal performance.

Therefore, it can be considered that the optical pickup for CD, DVD, andBD uses two different objective lenses: one is an objective lens ofdiffraction type used for the above-mentioned optical pickup for CD andDVD and the other is an objective lens dedicated to BD.

However, a thin optical disc drive in notebook personal computers, etc.,in particular, has a small space for mounting optical components, whichmakes it difficult to mount optical components of an optical pickupmounting two objective lenses.

Then, Reference 1 does not disclose any embodiment of optical componentsother than two objective lenses and actuators.

In Reference 2, optical beams are independently applied from a directionequivalent to the optical disc radial direction to objective lensesarranged in the optical disc tangential direction. In the case of a thindisc drive having a restriction on radial dimensions, it is not possibleto mount these components on the thin optical disc drive with theconfiguration of Reference 2. With the configuration in which objectivelenses are arranged in the optical disc tangential direction, either ofthese objective lenses will cause off-track. This causes a restrictionthat the Differential Push Pull Method (hereinafter referred to as DPP),one of the most generally used methods for detecting a tracking errorsignal (hereinafter referred to as TES), cannot be used.

Reference 3 discloses an optical pickup configuration in which twoobjective lenses are arranged in the tangential direction of an opticaldisc in a thin optical disc drive. Since optical paths for BD, DVD, andCD are once combined and then branched again directly under theobjective lenses, two branch elements are required. However, it isdifficult to realize desired transmission and reflection characteristicsusing a branch element for each of wavelengths of light beams used forCD, DVD, and BD. Also, the element is susceptible to an optical axisshift because the light beams pass through a common optical path.Moreover, since antireflection coating of optical components depend onthe wavelength of the entering optical beam, there arises a problem ofdegraded transmission factor of optical components arranged in thecommon optical path through which light beams for CD and those for BDpass, the light beam for CD being different in wavelength twice fromthat for BD. Furthermore, since the objective lenses are arranged in thetangential direction, there is a restriction that the DPP cannot beused.

An object of the present invention is to provide a thin optical pickupand an optical disc drive for three different media BD, DVD, and CD.

The above-mentioned object can be attained with a configurationdescribed in the appended claims of the present invention.

In accordance with the present invention, it is possible to provide athin optical pickup and an optical disc drive for three different mediaof BD, DVD, and CD.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, objects and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic diagram showing a configuration of an opticalpickup 001 according to a first embodiment of the present invention;

FIG. 2 is a schematic diagram showing an effect of tilted travelingdirections of BD and DVD optical beams according to the first embodimentof the present invention;

FIG. 3 is a schematic diagram showing a reason why two objective lensesmust be brought close to each other according to the first embodiment ofthe present invention;

FIG. 4 is a schematic diagram showing a difference between anR-direction incidence and a T-direction incidence according to the firstembodiment of the present invention;

FIG. 5 is a schematic diagram showing a configuration of an opticalpickup 200 according to a second embodiment of the present invention;

FIG. 6 is a schematic diagram showing a configuration of an opticalpickup 300 according to a third embodiment of the present invention;

FIG. 7 is a schematic diagram showing a configuration of an optical discdrive 400 according to a fourth embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In each embodiment of the present invention, an example will beexplained on the assumption of three different media BD, DVD, and CD.However, each embodiment of the present invention is not limited tothree different media BD, DVD, and CD, and application of the presentinvention to other optical discs such as HD-DVD causes no problem.Moreover, although the following explains the present invention indetail based on embodiments shown in the drawings, the present inventionis not limited by the explanations.

First Embodiment

A first embodiment of the present invention will be explained below indetail with reference to the accompanying drawings. The followingexplains an optical pickup mounted on a thin optical disc drive for BDand DVD.

FIG. 1 is a schematic diagram showing a configuration of an opticalpickup 001 according to the first embodiment. First of all, thefollowing explains a BD optical system.

An optical beam is emitted from a BD laser light source 002 as divergentlight. In order to write information to or read information from to BD,it is common to use a semiconductor laser with a wavelength from 395 nmto 415 nm. The BD laser light source 002 emits an optical beam with awavelength of about 405 nm. Moreover, a semiconductor laser generallyemits a linearly-polarized optical beam, and it is assumed that the BDlaser light source 002 also emits a linearly-polarized optical beam. Apath of the BD optical beam emitted from the BD laser light source 002is shown by a solid line 003. Dashed lines in line-symmetry with respectto the solid line 003 indicate the outermost circumference of thediffusion of the optical beam.

The optical beam emitted from the BD laser light source 002 enters a BDdiffraction grating 004. The optical beam is branched into a main lightbeam and two sub light beams by the BD diffraction grating 004 (paths ofsub light beams not shown). The two sub light beams are used to generatea DPP-based TES and a focal error signal (hereinafter referred to asFES) based on the Differential Astigmatic Detection (hereinafterreferred to as DAD). The DPP and DAD are well-known technologies andtherefore their explanations are omitted.

After passing through the BD diffraction grating 004, the optical beamtransmits through a polarization beam splitter 005, reflects off areflecting mirror 006, and enters an auxiliary lens 007. Aftertransmitting through the auxiliary lens 007, the optical beam isconverted into an approximately-parallel optical beam by a BD collimatelens 008 mounted on a lens holder 009 which is connected to a BDcollimate lens drive unit 011 through a shaft 010. The drawing assumesthat the BD collimate lens drive unit 011 uses a general stepping motor.By operating the shaft 010 and the lens holder 009 using the revolutionof this stepping motor as power, the mechanism of the unit 011 is suchthat the BD collimate lens 008 can be moved in a direction in parallelwith the optical beam entering the BD collimate lens.

After passing through the BD collimate lens 008, the optical beam entersa chromatic aberration correction element 013 which corrects chromaticaberrations caused by wavelength fluctuation of the BD laser lightsource 002 and temperature change of the optical pickup 001. Althougheither a combination of lenses or a lens of diffraction type isgenerally used as a chromatic aberration correction element, the use ofa lens of diffraction type is more effective for downsizing. Theintegration of the BD collimate lens 008 with the chromatic aberrationcorrection element 013, for example by making a diffraction groove forchromatic aberrations correction on the collimate lens, is effective fordownsizing with reduced number of components. Moreover, the use of sucha configuration causes no problem.

After passing through the chromatic aberration correction element 013,the optical beam enters a BD quarter wavelength plate 014 and isconverted into circularly-polarized light. After passing through the BDquarter wavelength plate 014, the optical beam reflects off a BDupward-reflection mirror 015 in the z direction, enters a BD objectivelens 016, and is converged onto the BD data layer (not shown). The BDobjective lens 016, mounted on an actuator 017, can be operated in the yand z directions in the drawing. The y direction is used for TES-basedcontrol and actuation at the time of lens shift, and the z direction forFES-based control.

For the BD objective lens 016, an area within an effective diameter ofthe lens is hatched.

After reflecting off the data layer, the optical beam passes through theBD objective lens 016, the BD upward-reflection mirror 015, thechromatic aberration correction element 013, the BD collimate lens 008,the auxiliary lens 007, the reflecting mirror 006, the polarization beamsplitter 005, and a BD detection lens 018, then reaches a BDPD 019. Whenthe BD detection lens 018 consists of a cylindrical lens and a sphericallens. When the light beam penetrates into the BD detection lens 018,predetermined astigmatism is given in a direction of about 45 degreesand used for detection of the FES. The function of this BD detectionlens 018 is to rotate the astigmatism in a desired direction as well asdetermine the size of the light spot converged onto the BDPD 019. Theoptical beam guided by the BDPD 019 is used for detection of aninformation signal recorded on the BD data layer and detection ofpositional control signals, such as TES and FES, for the light spotconverged onto the optical disc.

Then, an optical path ranging from the BD laser light source 002 to theBD data layer is referred to as a forward path, and an optical pathranging from the data layer to the BDPD 019 is referred to as a returnpath. The storage capacity of BD is about five times that of DVD, and aninformation pit of the BD data layer is smaller than that of DVD. Inorder to read this information spot, therefore, it is necessary that alight spot to the BD data layer be smaller than that to the DVD datalayer. The light spot strongly depends not only on the NA of objectivelens and the wavelength of laser light source but also on the forwardpath magnification (combined focal length of the auxiliary lens 007 andthe collimate lens 008 divided by the focal length of objective lens).The size of the converged light spot can be reduced by increasing thisforward path magnification. Therefore, it is necessary that the forwardpath magnification of the BD optical system be larger than that of theDVD optical system. When general semiconductor laser is used and thelight beam emitted therefrom is not to be shaped, it is preferable thatthe forward path magnification be multiplied by about 5 to 7 for DVD andby about 10 to 14 for BD.

When it is assumed that the auxiliary lens 007 is not used in the BDoptical system, the focal length of the BD collimate lens 008 must beincreased for a higher magnification, which makes it impossible to storethe BD laser light source 002 in the outer shape of the optical pickup.Therefore, the use of the auxiliary lens 007 is effective for downsizingthe total configuration while maintaining the effective forward pathmagnification.

Conversely, if the total configuration is downsized too much whilemaintaining the forward path magnification, the laser light source 002may come into contact with the BDPD 019 which are particularly largecomponents in the optical pickup, making the arrangement impossible.Then, it would be preferable that the focal length of the auxiliary lens007 and BD collimate lens 008 be set so that the laser light source 002may not come into contact with the BDPD 019 while maintaining theforward path magnification.

The reflecting mirror 006 is arranged so that the BD laser light source002 and BDPD 019 may not protrude from the outer shape and, as mentionedlater, the mirror 006 may not come into contact with the DVD opticalsystem.

For BD, an objective lens with a large NA (0.85) is used to reduce thesize of the light spot converged onto the BD data layer. However,spherical aberrations caused by a thickness error of the cover layerincrease in proportion to the 4th power of the NA. For BD, sincespherical aberrations caused by the thickness error of this cover layercannot be ignored, a mechanism for correcting spherical aberrations isrequired. In order to correct spherical aberrations, it is common tochange the form of the optical beam entering the objective lens fromparallelism to weak divergence and weak convergence. In the presentembodiment, means for changing the form of the optical beam entering theBD objective lens 016 from parallelism to weak divergence and weakconvergence is realized by arranging the BD collimate lens drive unit011 and moving the BD collimate lens 008 in the optical axis direction.

It is known that the dynamic range and correction sensitivity inspherical aberrations correction depend on the focal length of the BDcollimate lens 008. Specifically, a short focal length results in anarrow dynamic range and a high correction sensitivity, and a long focallength results in a wide dynamic range and a low correction sensitivity.The thickness error of the BD cover layer is specified by a standard,for example, the cover layers of 2-layer BD changes within a range from70 to 105 micrometers. If the focal length of the BD collimate lens 008is long, the dynamic range for correcting spherical aberrations causedby the thickness error of this cover layer increases, which is notsuitable for downsizing. Conversely, if the focal length is too short,the correction sensitivity becomes too high and accordingly actuation atfiner steps is required. Therefore, it would be preferable that thefocal length of the BD collimate lens 008 be within a range from about 9mm to 12 mm.

The following explains the DVD optical system.

An optical beam is emitted as divergence light from a DVD laser lightsource 050. In order to write information to or read information from toDVD, it is common to use a semiconductor laser with a wavelength of 660nm. The DVD laser light source 050 emits an optical beam with awavelength of about 660 nm. Moreover, it is assumed that the DVD laserlight source 050 also emits a linearly-polarized optical beam. The pathof the DVD optical beam emitted from the DVD laser light source 050 isshown by a solid line 051. Dashed lines in line-symmetry with respect tothe solid line 051 indicate the outermost circumference of the diffusionof the optical beam.

The optical beam emitted from the DVD laser light source 050 enters ahalf wavelength plate 052 and is converted into linearly-polarized lightin a predetermined direction, then enters a DVD diffraction grating 053.The optical beam is branched into a main light beam and two sub lightbeams by the DVD diffraction grating 053 (paths of sub light beams notshown). The two sub light beams are used to generate a DPP-based TES anda DAD-based FES.

After passing through the DVD diffraction grating 053, the optical beamreflects off a beam splitter 054 and is converted into anapproximately-parallel optical beam by a DVD collimate lens 055.

After passing through the DVD collimate lens 055, the optical beamenters a liquid crystal aberration correction element 056 having afunction for correcting coma aberrations in a predetermined direction,as mentioned in detail later. After passing through the liquid crystalaberration correction element 056, the optical beam enters a DVD quarterwavelength plate 057 and is converted into circularly-polarized light.

Although two lenses (DVD and BD objective lenses) are mounted, whenactually creating the optical pickup 001, an optimal tilt angle in eachof the optical disc radial and tangential directions may differ betweenthe DVD objective lens 059 and the BD objective lens 016. The liquidcrystal aberration correction element 056 is mounted to correct theshift of this optimal tilt angle. Since the shift of the tilt anglecorresponds to coma aberrations, the liquid crystal aberrationcorrection element 056 is mounted so that coma aberrations in theoptical disc radial and tangential directions be corrected.

After passing through the DVD quarter wavelength plate 057, the opticalbeam reflects off the DVD upward-reflection mirror 058 in the zdirection, enters the DVD objective lens 059, and is converged onto theDVD data layer (not shown). The BD objective lens 059, also mounted onan actuator 017, can be operated in the y and z directions in thedrawing.

Also for the BD objective lens 059, an area within an effective diameterof the lens is hatched in the drawing.

After reflecting off the data layer, the optical beam passes through theDVD objective lens 0059, a DVD upward-reflection mirror 058, the beamsplitter 054, and a DVD detection lens 060, then reaches a DVDPD 061.When the optical beam penetrates through the beam splitter 054,astigmatism is given for detection of the FES. The function of the DVDdetection lens 060 is to rotate the astigmatism in a desired directionas well as determine the size of the light spot converged onto the DVDPD061. The optical beam guided by the DVDPD 061 is used for detection ofthe information signal recorded on the DVD data layer and detection ofpositional control signals, such as TES and FES, for the light spotconverged onto the DVD data layer.

Moreover, a dotted line 070 in the x direction agrees with the opticaldisc radial direction and the optical pickup seek direction. Moreover,as shown in the drawing, the topside corresponds to the optical discouter circumferential direction and the bottom side the optical discinner circumferential direction.

The following describes the fact that the BD and DVD optical beams arelarger than the effective diameter of the objective lenses as shown inFIG. 1. In an optical disc drive, when writing information to or readinginformation from a predetermined track, the information is accessed notonly by seeking the optical pickup in the optical disc radial directionbut also by moving the objective lenses in the optical disc radialdirection by operating the actuator mounting the objective lenses.Moving the objective lenses in the optical disc radial direction byoperating the actuator is referred to as a lens shift. A lens shift ofabout ±0.3 mm is common. Specifically, the optical beam entering anobjective lens must be larger than the effective diameter of theobjective lens by at least about ±0.3 mm in the optical disc radialdirection.

For the above reason, the BD and DVD optical beams are larger than theeffective diameter of the objective lenses in FIG. 1.

Then, as shown in FIG. 1, the traveling direction of the optical beamentering the DVD upward-reflection mirror 058 is tilted by an angle of θwith respect to that of the optical beam entering the BDupward-reflection mirror 015. This arrangement is used since a mutualinterference between BD and DVD optical components is avoided.

FIG. 2 shows another case of the optical pickup 001 in which thetraveling directional angle of the optical beam entering the BDupward-reflection mirror 015 is the same as that of the optical beamentering the DVD upward-reflection mirror 058, i.e., the angle θ shownin FIG. 1 is zeroed. As shown in FIG. 2, there is an area where the BDcollimate lens 008, chromatic aberration correction element 013, andquarter wavelength plate overlap the DVD optical beam. Moreover, whenthe BD collimate lens 008 operates in the incidence optical axisdirection, a movable range of the BD collimate lens 008 and lens holder009 is shown as an area 012 hatched with dots. If the BD collimate lens008 is operated as shown in the drawing, it may overlap the DVD opticalbeam in a remarkable range. Therefore, it is not possible to install awall between the BD and DVD optical beams, arising a problem ofinsufficient strength of the optical pickup.

For this reason, the traveling direction of the optical beam enteringthe DVD upward-reflection mirror 058 is tilted by an angle of θ withrespect to that of the optical beam entering the BD upward-reflectionmirror 015.

Moreover, if the DVD collimate lens 055 is arranged directly under theDVD upward-reflection mirror 058, an area overlapping the BD opticalcomponents cannot be avoided. Therefore, the DVD collimate lens 055 isarranged further in the x direction than the BD collimate lens 008 asshown in the drawing, thus avoiding areas where each of DVD and BDoptical component overlaps.

For example, although it seems that the angle θ can be zeroed byarranging the BD objective lens 016 and DVD objective lens 059 with agap in the optical disc radial direction, the angle cannot actually bezeroed. If the gap in the optical disc radial direction between the BDobjective lens 016 and the DVD objective lens 059 is minimized, there isan advantage that the performance of the actuator can be improved. Ifthe gap in the optical disc radial direction between the BD objectivelens 016 and the DVD objective lens 059 is large, it would be impossibleto read information from or write information to the outermost andinnermost circumferences of the optical disc.

FIG. 3 shows a thin optical disc drive 101 for notebook personalcomputers, mounting the optical pickup 001. FIG. 3A shows a case whenthe optical pickup 001 is arranged at the outermost circumference, andFIG. 3B a case when it is arranged at the innermost circumference.

With the optical disc drive 101, an optical disc 080 is fixed to aspindle 071 for rotating an optical disc 080. Moreover, theconfiguration of the optical pickup 001 is such that it can access theoptical disc in the radial direction along two guide bars 072.

When reading the DVD outermost circumference, the optical beam emittedfrom the optical pickup 001 must be applied to the track of theoutermost circumference of the optical disc 080. Therefore, the opticalpickup 001 must access the outermost circumference so that the DVDobjective lens 059 of the optical pickup 001 be positioned exactly atthe track of the outermost circumference of the optical disc 080 asshown in FIG. 3A.

Thus, unless the DVD objective lens 059 can be positioned exactly at thetrack of the outermost circumference of the optical disc 080 when theoptical pickup 001 accesses the outermost circumference, the informationin the outermost circumference cannot be read.

Moreover, when reading the BD innermost circumference, the optical beamemitted from the optical pickup 001 must be applied to the track at theinnermost circumference of the optical disc 080. Therefore, the opticalpickup 001 must access the innermost circumference so that the BDobjective lens 016 of the optical pickup 001 be positioned exactly atthe track at the innermost circumference of the optical disc 080 asshown in FIG. 3B.

Thus, unless the DVD objective lens 016 can be positioned exactly at thetrack at the innermost circumference of the optical disc 080 when theoptical pickup 001 accesses the innermost circumference, the informationin the innermost circumference cannot be read.

When two objective lenses are arranged in the optical disc radialdirection as mentioned above, the gap between the two objective lensesmust be minimized so that both objective lenses can read the informationsignals at the innermost and outermost circumferences. With the gapbetween the two objective lenses arranged in this manner, if the angle θbetween the traveling directional angle of the optical beam entering theBD upward-reflection mirror 015 and that of the optical beam enteringthe DVD upward-reflection mirror 058 is zeroed, i.e., the two opticalbeams are in parallel as shown in FIG. 2, there arises a problem ofinterference between BD and DVD optical components. Therefore, thetraveling direction of the optical beam entering the DVDupward-reflection mirror 058 is tilted by an angle of θ with respect tothat of the optical beam entering the BD upward-reflection mirror 015 asshown in FIG. 1.

Moreover, although the general configuration of the optical pickup 001is such that it accesses in the 45-degree radial direction of theoptical disc drive 101 in the drawing, giving a slight angle withrespect to the radial direction as shown in the drawing is alsoeffective for increasing the optical mounting space of the opticalpickup.

Then, when two objective lenses are arranged in the optical disc radialdirection, another possible configuration is such that, instead ofapplying the optical beam from the same direction to the two objectivelenses as shown in FIG. 1, the BD optical beam is applied from theoptical disc radial direction.

FIG. 4 is a diagram showing a problem which may arise when an opticalbeam is applied from the optical disc radial direction. FIG. 4A is adiagram showing the objective lenses of the optical pickup and thedirection of the optical beam applied thereto. FIG. 4B is a diagramshowing a cross section of the upward-reflection mirror and theobjective lens when the optical beam is applied from the optical disctangential direction (T-direction incidence). FIG. 4C is a diagramshowing a cross section of the upward-reflection mirror and theobjective lens when the optical beam is applied from the optical discradial direction (R-direction incidence).

As shown in FIG. 4A, the optical pickup 110 incorporates two objectivelenses 111 and 112 arranged along a dotted line 070 which agrees withthe optical disc radial direction. Application of the optical beam tothe objective lens 111 from the optical disc tangential direction(direction indicated by an arrow 113) is referred to as T-directionincidence, and application of the optical beam to the objective lens 112from the optical disc radial direction (direction indicated by an arrow114) is referred to as R-direction incidence.

In FIG. 4B and FIG. 4C, a dotted line 115 indicates an upper heightlimit of the optical pickup, a dotted line 116 a lower height limit ofthe optical pickup, a dotted line 117 an upper operating limit of theobjective lenses 111 and 112 in the height direction, and a dotted line118 a lower operating limit of the objective lenses 111 and 112 in theheight direction.

In the case of T-direction incidence in FIG. 4B, the direction of lensshift of the objective lens 111, i.e., the optical disc radialdirection, agrees with a direction perpendicular to the paper surface ofthe drawing (Y direction in the drawing). Therefore, in the case of theT-direction incidence, no margin of optical beam is required for lensshift.

In the case of the R-direction incidence shown in FIG. 4C, conversely,the direction of lens shift of the objective lens 112, i.e., the opticaldisc radial direction, agrees with the horizontal direction of the papersurface of the drawing (Y direction in the drawing). Therefore, a marginof optical beam is required for lens shift. This is because the opticalbeam effective diameter shown in FIG. 4C is larger than that shown inFIG. 4B.

Specifically, in connection with the height direction of the opticalpickup, the T-direction incidence as shown in FIG. 4B is moreadvantageous, i.e., has an advantage that the height of the opticalpickup can be reduced.

Moreover, the T-direction incidence has another advantage that a wideoperating range of objective lenses can be secured because of a smalleffective diameter.

For the R-direction incidence, dimensions of components in the heightdirection of the optical pickup are large, which results in an increasein the number of defective outer shapes at the time of optical pickupmass production, which may remarkably reduce the adaptability to massproduction. The adaptability to mass production is a major factor whendetermining the cost of the optical pickup. Specifically, theT-direction incidence has a major advantage of high adaptability to massproduction because of a wide margin in the height direction of theoptical pickup.

Then, as explained so far, the optical pickup 001 of the presentinvention includes a first laser light source which emits an opticalbeam with a first wavelength, i.e., the DVD laser light source 050; asecond laser light source which emits an optical beam with a secondwavelength shorter than the wavelength of the first laser light source,i.e., the BD laser light source 001; a first collimate lens whichconverts the optical beam emitted from the DVD laser light source 050into an approximately-parallel optical beam, i.e., the DVD collimatelens 055; a second collimate lens which converts the optical beamemitted from the BD laser light source 001 into anapproximately-parallel optical beam, i.e., the BD collimate lens 008; afirst objective lens which converges the approximately-parallel opticalbeam from the DVD collimate lens 055 to the optical disc, i.e., the DVDobjective lens 059; and a second objective lens which converges theapproximately-parallel optical beam from the BD collimate lens 008 tothe optical disc, i.e., the BD objective lens 016; wherein the DVDobjective lens 055 and BD objective lens 016 are arranged in the opticaldisc radial direction, i.e., a direction which agrees with the dottedline 070, and the BD objective lens 016 is arranged closer to the sideof the optical disc outer circumference than the DVD objective lens 059.

Moreover, since the DVD collimate lens 055 is arranged on right-handside when the BD objective lens 016 is viewed from the DVD objectivelens 059, the BD collimate lens 008 is arranged on the left-hand sidewhen the DVD objective lens 059 is viewed from the BD objective lens016.

Moreover, the gap between the DVD collimate lens 055 and the DVDobjective lens 059 is made longer than the gap between the BD collimatelens 008 and the BD objective lens 016.

Furthermore, the traveling direction of the optical beam entering theDVD collimate lens 055 and that of the optical beam entering the BDcollimate lens 008 forms an angle of 0 to 15 degrees.

Moreover, a collimate lens drive unit for operating the BD collimatelens 008, i.e., the BD collimate lens drive unit 011 is arranged in adirection which is in parallel with the traveling direction of theoptical beam entering the BD collimate lens 008. The BD collimate lensdrive unit 011 is arranged closer to the side of the optical disc outercircumference than the BD objective lens 016.

By arranging the optical components of the optical pickup in thismanner, it is possible to offer an optical pickup to be mounted on athin optical disc drive.

Moreover, it is also an advantage of the optical pickup of the presentinvention that the conventional general DPP can be used for both opticalsystems by arranging two objective lenses in the optical disc radialdirection.

Moreover, the use of completely independent components for the BD andDVD optical systems can eliminate common optical components andtherefore is effective for improving the transmission efficiency ofoptical components.

Although the first embodiment assumes the astigmatic detection fordetection of FES and the DPP for detection of TES, the use of othermethods, for example, a spot size method for detection of FES and acombination with DPP for detection of TES causes no problem.

When accessing predetermined radial positional information on theoptical disc, the optical pickup is moved in the optical disc radialdirection and therefore inertia force occurs in the BD collimate lens inthe direction opposite to the direction of movement of the opticalpickup. Specifically, if there is a component of the radial direction ofthe optical disc in the movable direction of the BD collimate lens, eachtime the optical pickup accesses information, the BD collimate lens ismoved in the optical disc radial direction by inertia force. Thus, ifthe BD collimate lens moves each time the optical pickup accessesinformation, spherical aberrations correction must be performed eachtime the optical disc drive accesses information, resulting in a longwrite and read processing time of the optical disc drive.

In order to avoid movement of the BD collimate lens in the radialdirection by this inertia force, it would be preferable that the BDcollimate lens is arranged so that its movable direction beperpendicular to the optical disc radial direction and a restriction begiven in the optical disc radial direction so that the lens does notmove even if force is applied.

For this reason, in FIG. 1, the traveling direction of the optical beamtraveling between the BD collimate lens 008 and the BD objective lenses016 is made in agreement with a direction perpendicular to the opticaldisc radial direction, and the traveling direction of the optical beamtraveling between the DVD collimate lens 055 and the DVD objectivelenses 059 is tilted by an angle of θ with respect to a directionperpendicular to the optical disc radial direction.

In the DVD optical system, since optical components are fixed withadhesive agent or the like and there are no operating optical componentssuch as the BD collimate lens 008, no optical components are moved byinertia force.

Second Embodiment

A second embodiment of the present invention will be explained below indetail with reference to the accompanying drawings. The followingexplains an optical pickup mounted on a thin optical disc drive for BD,DVD, and CD.

FIG. 5 is a schematic diagram showing a configuration of an opticalpickup 200 according to the second embodiment. Since the optical pickup200 according to the second embodiment uses the same BD optical systemof the optical pickup 001 of the first embodiment, their explanationsare omitted. Unlike the optical pickup 001 according to the firstembodiment, the optical pickup 200 according to the second embodiment isapplied to CD and therefore the optical pickup 200 mounts atwo-wavelength multi-laser light source 201 instead of the DVD laserlight source 050.

The two-wavelength multi-laser is a laser light source mounting in itscase two laser chips which emit optical beams with differentwavelengths. It is common that semiconductor laser with a wavelength of660 nm is used to write information to or read information from to DVD,and semiconductor laser with a wavelength of 780 nm to write informationto or read information from to CD. Therefore, two-wavelength multi-laser201 mounts a DVD laser chip 202 which emits an optical beam with awavelength of about 660 nm as well as a CD laser chip 203 which emits anoptical beam with a wavelength of about 780 nm. Then, the followingexplains the DVD optical system.

A DVD optical beam is emitted as divergence light from the DVD laserchip 202 of the two-wavelength multi-laser 201. Moreover, it is assumedthat the DVD laser chip 202 also emits a linearly-polarized opticalbeam. The path of the DVD optical beam emitted from the DVD laser chip202 is shown by a solid line 204. Dashed lines in line-symmetry withrespect to the solid line 204 indicate the outermost circumference ofthe diffusion of the optical beam.

The optical beam emitted from the DVD laser chip 202 enters a widebandhalf wavelength plate 205 and is converted into linearly-polarized lightin a predetermined direction. When optical beams with a wavelength ofabout 660 nm and a wavelength of about 780 nm enter the wideband halfwavelength plate 205, the wideband half wavelength plate 205 functionsas a half wavelength plate to both wavelengths. This half wavelengthplate is a common optical element for existing DVD/CD compatible opticalpickups.

The optical beam enters a wavelength selective diffraction grating 206.When an optical beam with a wavelength of about 660 nm enters thewavelength selective diffraction grating 206, the optical beam isbranched with diffraction angle θ DVD; when an optical beam with awavelength of about 780 nm enters it, the optical beam is branched withangle θ CD which is different from diffraction angle θ DVD. Such awavelength selective diffraction grating can be manufactured byarranging the groove depth and refractive index of the diffractiongrating and is used for an optical pickup mounting a recenttwo-wavelength multi-laser light source. Then, the optical beam isbranched into a main light beam and two sub light beams by thewavelength selective diffraction grating 206 (paths of sub light beamsnot shown). The two sub light beams are used to generate DPP and DADsignals.

After passing through the wavelength selective diffraction grating 206,the optical beam reflects off a beam splitter 207 and then convertedinto an approximately-parallel optical beam by a collimate lens 208.

After passing through the collimate lens 208, the optical beam enters aliquid crystal aberration correction element 209 having a function forcorrecting coma aberrations in a predetermined direction for the DVDoptical beam. Moreover, patterns are arranged also for the CD opticalbeam so that coma aberrations be corrected like DVD, with a differentamount of correction.

After passing through the liquid crystal aberration correction element209, the optical beam enters a wideband quarter wavelength plate 210 andis converted into circularly-polarized light. The wideband quarterwavelength plate 210 is also an optical component which functions as aquarter wavelength both for the DVD and CD optical beams.

After passing through the wideband quarter wavelength plate 210, theoptical beam reflects off an upward-reflection mirror 211 in the zdirection, enters a DVD/CD compatible objective lens 212, and isconverged onto the DVD data layer (not shown). The DVD/CD compatibleobjective lens 212, mounted on an actuator 017, can be operated in the yand z directions in the drawing. Also for the DVD/CD compatibleobjective lens 212, an area within an effective diameter of the lens ishatched in the drawing. Moreover, for the DVD/CD compatible objectivelens as mentioned above, the DVD and CD wavelengths, NA, and cover layerthickness can be corrected through the use of objective lenses ofdiffraction type.

After reflecting off the data layer, the optical beam passes through theDVD/CD compatible objective lens 212, the upward-reflection mirror 211,the wideband quarter wavelength plate 210, the liquid crystal aberrationcorrection element 209, the collimate lens 208, the beam splitter 207,and a detection lens 213, then reaches a PD 214. When the optical beampenetrates through the beam splitter 213, astigmatism is given fordetection of the FES. The function of the detection lens 213 is torotate the astigmatism in a desired direction as well as determine thesize of the light spot converged onto the PD 214. The optical beamguided by the PD 213 is used for detection of the information signalrecorded on the DVD data layer and detection of positional controlsignals, such as TES and FES, for the light spot converged onto the DVDdata layer.

Moreover, a dotted line 070 in the x direction agrees with the opticaldisc radial direction as well as the optical pickup seek direction.Moreover, as shown in the drawing, the topside corresponds to theoptical disc outer circumferential direction and the bottom side theoptical disc inner circumferential direction.

Although two lenses (the DVD/CD compatible objective lens 212 and the BDobjective lens) are mounted, when actually creating the optical pickup200, an optimal tilt angle in each of the optical disc radial andtangential directions may differ between the DVD/CD compatible objectivelens 059 and the BD objective lens 016. A liquid crystal aberrationcorrection element 056 is mounted to correct the shift of this optimaltilt angle. Since the shift of the tilt angle corresponds to comaaberrations, the liquid crystal aberration correction element 056 ismounted so that coma aberrations in the optical disc radial andtangential directions be corrected.

The following explains the CD optical system.

A CD optical beam is emitted as divergence light from the CD laser chip203 of the two-wavelength multi-laser 201. Moreover, it is assumed thatthe CD laser chip 203 also emits a linearly-polarized optical beam.Since the path of the CD optical beam emitted from the CD laser chip 203is almost the same as that for DVD, the CD optical path itself isomitted assuming that the solid line 204 agrees with the CD path.

The optical beam emitted from the CD laser chip 203 enters the widebandhalf wavelength plate 205 and is converted into linearly-polarized lightin a predetermined direction.

Then, the optical beam enters the wavelength selective diffractiongrating 206 and is branched into a main light beam and two sub lightbeams with diffraction angle θ CD which is different from diffractionangle θ DVD (paths of sub light beams not shown). The two sub lightbeams are used to generate DPP and DAD signals.

After passing through the wavelength selective diffraction grating 206,the optical beam reflects off the beam splitter 207 and is convertedinto an approximately-parallel optical beam by the collimate lens 208.

After passing through the collimate lens 208, the optical beam entersthe liquid crystal aberration correction element 209 having a functionfor correcting coma aberrations in a predetermined direction also forthe CD optical beam.

After passing through the liquid crystal aberration correction element209, the optical beam enters the wideband quarter wavelength plate 210and is converted into circularly-polarized light.

After passing through the wideband quarter wavelength plate 210, theoptical beam reflects off the upward-reflection mirror 211 in the zdirection, enters the DVD/CD compatible objective lens 212, and isconverged onto the CD data layer (not shown).

After reflecting off the data layer, the optical beam passes through theDVD/CD compatible objective lens 212, the upward-reflection mirror 211,the wideband quarter wavelength plate 210, the liquid crystal aberrationcorrection element 209, the collimate lens 208, the beam splitter 207,and the detection lens 213, then reaches the PD 214. When the opticalbeam penetrates through the beam splitter 213, astigmatism is given,like DVD, for detection of the FES. The function of the detection lens213 is to rotate the astigmatism of the CD optical beam in a desireddirection, like the DVD optical beam, as well as determine the size ofthe light spot converged onto the PD 214. The optical beam guided by thePD 213 is used for detection of the information signal recorded on theCD data layer and detection of positional control signals, such as TESand FES, for the light spot converged onto the CD data layer.

It is possible to easily provide a thin optical pickup for threedifferent media BD, DVD, and CD based on the optical pickup 001according to the first embodiment using the two-wavelength multi-laseras the DVD optical system, like the optical pickup 200, and mountingabove-mentioned optical components.

Moreover, the use of completely independent components for the BD andDVD optical systems with different two-wavelengths can eliminate commonoptical components and therefore is effective for improving thetransmission efficiency of optical components.

Third Embodiment

A third embodiment of the present invention will be explained below indetail with reference to the accompanying drawings. The followingexplains a variation of the second embodiment.

FIG. 6 is a schematic diagram showing an optical pickup 300 in a thinoptical disc drive for BD, DVD and CD.

Regarding the optical pickups 001 according to the first embodiment andthe optical pickup 200 according to the second embodiment, a combinationof the DPP and DAD has been explained as a method for detecting TES andFES for BD. For the third embodiment, the use of the PP method fordetection of TES and a knife edge for detection of FES will be explainedas a variation of the above-mentioned method.

The optical pickup 300 is a modified version of the optical pickup 200:the BD diffraction grating 003, the detection lens 005, and the BDquarter wavelength plate 014 of the optical pickup 200 have beenremoved; a multi-functional element 301, and a PD 302 with a differentlight-sensitive area pattern in PD from the PD 019 added; and thecollimate lens drive element 011 replaced with a BD collimate lens driveelement 312 formed of a piezoelectric element. Since the DVD and CDoptical systems are not modified in particular, their explanations areomitted.

Taking notice of the above-mentioned modified points, the BD opticalsystem will be explained below.

An optical beam of the 405-nm band is emitted as divergence light oflinearly-polarized light from the BD laser light source 002. The opticalbeam emitted from the BD laser light source 002 passes through apolarization beam splitter 005, a reflecting mirror 006, and anauxiliary lens 007, then is converted into an approximately-paralleloptical beam by a BD collimate lens 008. The BD collimate lens 008 ismounted on a lens holder 310 which is connected with a BD collimate lensdrive unit 312, where a BD collimate lens drive unit 011 using apiezoelectric element.

This BD collimate lens drive element 314 mounts a piezoelectric element314 which expands and contracts in the x direction in the drawing byapplying a voltage. By operating a lens holder 313 using expansion andcontraction of the piezoelectric element, the BD collimate lens 008 canbe operated in a direction in parallel with the optical beam enteringthe BD collimate lens 008. The lens holder is fixed to a shaft 313 andsupported by a shaft 312 to realize stable operation of the BD collimatelens. Generally, the use of a piezoelectric element is more advantageousfor downsizing than the use of a stepping motor. Therefore, the use of apiezoelectric element for the collimate lens drive unit has an advantagethat the optical pickup can be downsized.

After passing through the BD collimate lens 008, the optical beam passesthrough a chromatic aberration correction element 013 and then entersthe multi-functional element 301 which is a compact element formed bysticking a polarization diffraction grating to a quarter wavelengthplate. The polarization diffraction grating diffracts thelinearly-polarized optical beam in a predetermined direction, andtransmits the linearly-polarized optical beam in a directionperpendicular to the predetermined direction. Therefore, themulti-functional element 301 mounting a polarization diffraction gratingand a quarter wavelength plate transmits the optical beam traveling fromright to left on the paper surface and diffracts the optical beamtraveling from left to right on the paper surface. Specifically, theoptical beam coming from the chromatic aberration correction elementpasses through the area of the polarization diffraction grating of themulti-functional element 301 and is converted into circularly-polarizedlight by the quarter wavelength plate. The optical beam returned tocircularly-polarized light by the multi-functional element 301 reflectsoff a BD upward-reflection mirror 015 in the z direction, enters a BDobjective lens 016, and is converged onto the BD data layer (not shown).

After reflecting off the data layer, the optical beam enters the BDobjective lens 016, the BD upward-reflection mirror 015, and themulti-functional element 301. After entering the multi-functionalelement 301, the optical beam is converted into linearly-polarized lightin a direction perpendicular to the forward path fromcircularly-polarized light in the area of the quarter wavelength plate,then branched into a plurality of optical beams in the area of thepolarization diffraction grating. As long as the knife edge can be usedfor detection of FES and the PP method for detection of TES, any desiredpatterns are acceptable as a grating groove pattern for the polarizationdiffraction grating. Moreover, since the knife edge and PP method arealso well-known methods, their explanations are omitted. The opticalbeam branched into a plurality of optical beams by the multi-functionalelement 301 passes through the chromatic aberration correction element013, the BD collimate lens 008, the auxiliary lens 007, the reflectingmirror 006, and the polarization beam splitter 005, then reaches a BDPD302. The optical beam guided by the BDPD 302 is used for detection ofthe information signal recorded on the BD data layer and detection ofpositional control signals, such as TES and FES, for the light spotconverged onto the optical disc. Moreover, since an assumed detectionmethod is different from that of the optical pickups 001 and 200, thelight-sensitive area pattern is different from that of a BDPD 019.However, as long as the above methods can be used, any desiredlight-sensitive area patterns are acceptable.

As mentioned above, the use of the present invention makes it possibleto use detection methods different from those of the optical pickup 100,like the optical pickup 300, realizing a variety of optical pickups.

Although a combination of the knife edge and PP method has beenexplained above, the present invention can also be applied to a casewhen optical components has been modified, for example, by using thespot size method together with the PP method.

Fourth Embodiment

In a fourth embodiment, an optical disc drive 400 mounting an opticalpickup for three different media CD, DVD, and BD explained in theabove-mentioned embodiments will be explained.

FIG. 7 is a diagram showing the optical disc drive 400 mounting anoptical pickup 403 and a schematic circuit configuration of the opticaldisc drive 400.

The optical disc drive 400 incorporates an optical disc 405 fixed to aspindle 401 which rotates the optical disc 405.

Moreover, the optical disc drive 400 incorporates a guide bar 402 alongwhich the optical pickup 403 can access a predetermined radial positionof an optical disc 400.

A host 425 indicates an information appliance using an optical discdrive, such as a personal computer. When an instruction for readinginformation on the optical disc 405 is inputted from the host 425 to acontrol circuit 412 in the optical disc drive 400, the control circuitactivates a spindle motor drive circuit 419 to operate the spindle 401,starting revolution of the optical disc 405.

Then, the control circuit 412 activates a laser light source switchingcircuit to drive a DVD laser light source in an optical pickup 402.Then, the control circuit 412 activates a laser light source controlcircuit 418 to turn on the DVD laser light source by the reading power.

Then, the control circuit 412 activates an actuator drive circuit 415 tooperate an actuator in the optical pickup 402 in the height direction. Asignal detected from a PD of the optical pickup 403 is transmitted to aservo signal generation circuit 410, and a FES is generated from thedetected signal. The control circuit 412 activates an optical disc typedetermination circuit 413 to determine the medium type of the opticaldisc 405 based on the FES. Since each of BD, DVD, and CD has a differentcover layer thickness, the medium type can be determined by detectingfrom the FES the gap between the data layer and the surface of theoptical disc 405, i.e., the cover layer thickness.

The reason why the DVD laser light source is first turned on is thatamplitude degradation of the FES caused by spherical aberrations can beminimized regardless of whether the optical disc 405 is CD or BD becausethe cover layer thickness of DVD is intermediate in comparison with thatof CD and BD. For example, the amplitude of the FES detected from CDunder the BD read condition is very small because of the influence ofspherical aberrations, making it difficult to detect the CD data layer.

A control circuit 412 activates a laser switching circuit 417 to drive alaser light source corresponding to the optical disc determined by theoptical disc type determination circuit 413. Then, the control circuit412 activates the laser light source control circuit 418 to turn on thelaser light source by the reading power.

Then, the control circuit 412 activates the actuator drive circuit 415to operate again the actuator in the optical pickup 402 in the heightdirection. The signal detected from the PD of the optical pickup 403 isretransmitted to the servo signal generation circuit 410 to generateservo signals FES and TES. The generated servo signals are transmittedfrom the control circuit 412 to the actuator drive circuit 415 asrequired, to operate the actuator in the optical pickup 403, performpositional control of objective lenses, and converge optical beams to apredetermined data layer. The control circuit 412 also has a functionfor activating an access control circuit 414 to move the optical pickup403 to a predetermined radial position along the guide bar 402.

Then, after converging optical beams to the data layer of the opticaldisc 405, the detected signal from the PD in the optical pickup 403 istransmitted to an information signal playback circuit 4130. Theinformation signal playback circuit 4130 reads an information signalrecorded on the optical disc 405 from the above-mentioned detectedsignal and outputs the information signal to the host 425.

For example, if the optical disc 405 is a BD, the control circuit 412also activates a BD collimate lens drive circuit 416 to performspherical aberrations correction so as to maximize the readingperformance (for example, jitter and amplitude of detected signal) forthe information signal generated from the above-mentioned informationsignal playback circuit 4130.

By activating the circuit of the optical disc drive 400 as mentionedabove, the host 425 can acquire desired read information.

Then, if an instruction for recording information to the optical disc405 is inputted from the host 425 to the control circuit 412, thecontrol circuit performs the same operation as the above-mentioned readoperation and then turns on a laser light source which is suitable forthe optical disc 405 to converge the optical beam onto the optical disc405.

Then, recording information is inputted from the host 425 to a recordinginformation signal conversion circuit 420, then is converted into arecording signal which is suitable for a predetermined medium by therecording information signal conversion circuit 420. This recordingsignal is transmitted to the control circuit 412. Then, the controlcircuit 412 activates the laser light source control circuit 418 toperform power control of the laser light source and writes the recordingsignal to the optical disc 405. At this time, the control circuit 412activates the access control circuit 414 and the spindle motor drivecircuit 419 to perform access control of the optical pickup 402 androtational control of an optical disc 401 according to the recordingsignal.

By activating the circuit of the optical disc drive 400 as mentionedabove, the recording information received from the host can be writtento the optical disc 405.

Although the embodiment of the optical disc drive 400 has been explainedabove, the present invention is not limited to this embodiment as longas at least the BD collimate lens drive circuit 416 and the laser lightsource switching circuit 417 are mounted.

While we have shown and described several embodiments in accordance withour invention, it should be understood that disclosed embodiments aresusceptible to changes and modifications without departing from thescope of the invention. Therefore, we do not intend to be bound by thedetails shown and described herein but intend to cover all such changesand modifications as falling within the ambit of the appended claims.

1. A thin optical pickup which exchanges information with an opticaldisc by means of optical beams, the thin optical pickup comprising: afirst laser light source which emits an optical beam with a firstwavelength; a second laser light source which emits an optical beam witha second wavelength shorter than the first wavelength; a first collimatelens which adjusts the optical beam emitted from the first laser lightsource; a second collimate lens which adjusts the optical beam emittedfrom the second laser light source; a first objective lens whichconverges the optical beam from the first laser light source to anoptical disc; a second objective lens which converges the optical beamfrom the second laser light source to an optical disc; and a auxiliarylens which changes the form of the optical beam from the second laserlight source to divergence and convergence; wherein a first opticalsystem comprises the first laser light source, the first collimate lens,and the first objective lens; a second optical system comprises thesecond laser light source, the auxiliary lens, the second collimatelens, and the second objective lens; the first and second objectivelenses are arranged in a radial direction of the optical disc; theauxiliary lens is arranged between the second collimate lens and thesecond laser light source.
 2. The thin optical pickup according to claim1, the first optical system and the second optical system are arrangedon a common side of a radial line passing through the first objectivelens and the second objective lens.
 3. The thin optical pickup accordingto claim 1, the auxiliary lens is concave lens.
 4. The thin opticalpickup according to claim 1, the forward path magnification of thesecond optical system be larger than that of the first optical system.5. The thin optical pickup according to claim 1, the forward pathmagnification be multiplied by about 5 to 7 for the first optical systemand by about 10 to 14 for the second optical system.
 6. The thin opticalpickup according to claim 1, the thin optical pickup comprises sphericalaberrations correction unit.